Utilizing dogs and baboons, we will subject the animals to sterile surgery in which we will implant devices to measure arterial pressure, ventricular pressure, ventricular filling pressure and aortic flow. We will also implant electrodes to stimulate the atrium and ventricle, and cuffs which enable us to decrease the ventricular filling pressure or increase the arterial pressure. After recovery, the animals will be studied without anesthesia, or with light morphine. We will alter the filling pressure while maintaining constant arterial pressure and stimulating the heart at a constant rate to determine the direct effects of filling pressure on stroke volume. We will then increase the arterial pressure with the aortic cuff, holding the filling pressure constant. This will give us in the intact animal a coefficient for direct effects of filling pressure and arterial pressure on the stroke volume. Animals will then be studied under sympathetic influence, during exercise, etc. We will also wish to see the relative importance of peripheral resistance change and cardiac changes in both the dog and baboon, and these procedures will make that possible. Finally, we will study the possibility that low pressure baroreceptors influence the circulation and the importance of venoconstriction in circulatory regulation.